Source: NMR Short Course, SPWLA 36th Annual Symposium, June, 1995, Paris
As suggested in the course notes, the problem was posed that typical log suites can not be used to interpret grain size. In contrast, the Magnetic Resonance Imaging Log (MRIL) can distinguish between producible water and irreducible water which is related to grain size. In this sandstone example, identified in the short course only as a sandstone from the U.S. Gulf Coast, the pattern of decreasing resistivity with depth appears to be a transition zone. However, the interpretation of the MRIL indicates no transition zone. Rather, the MRIL indicates only decreasing grain size with depth which is responsible for increasing amounts of bound water.
PfEFFER analysis utilizes Pickett cross plots with points connected that are annotated in color with depth, gamma ray, and MBVI. The cross plots are annotated with MRIL’s irreducible bulk volume water (MBVI). The MBVI in this sandstone ranges from 3% at the top to 6% at the base while porosity remains nearly constant. In contrast, the BVW contours on the Pickett cross plot show values ranging from 7% at the top of the sandstone increasing to 30% at the base. The minimum bulk volume water at the top of the section is similar same to the irreducible (3% and 7%), but the difference between the two methods is large at the base of the sandstone (30% from the Pickett cross plot and 6% from the MRIL). This suggests that the resistivity and porosity logs are responding to a transition zone where water saturation and BVW are increasing while porosity remains constant. The increasing deviation of BVW and MBVI are typical of a transition zone. No clustering of points on the Pickett cross plot suggests that the entire sandstone lies within a long transition zone.
The MRI derived permeability equals 5 darcies. The Timur equation can be used to estimate permeability for clean quartz sandstones. The Timur relationship in this case resulted in the same permeability value when water saturation and porosity values were used that are associated with the minimum BVW (rightmost point on the Pickett cross plot). The correspondence suggests that the minimum BVW is close to irreducible which it is according to the match with the MRIL bound water determination. The Timur permeability contours can be shown on the Pickett cross plot with the limitation that values are valid for sandstones at irreducible water saturation.